Production of embryos by oocyte cytoplast–blastomere fusion in domestic animals

Abstract

Embryos of amphibians, sheep, cattle, pigs and rabbits have been multiplied by nuclear transfer. Successful nuclear transfer in these species has been accomplished by transfer of a blastomere from a late-stage embryo into an enucleated oocyte with large scale multiplication by repeating the procedure using blastomeres from the embryos produced from nuclear transfer. This allows the production of clonal lines which, when appropriately selected for performance in a given trait, can be reproduced to capture in the offspring expression of additive and non-additive inheritance. The efficiency of these procedures is high only for amphibian embryos for which as many as 1000 offspring can be made from a blastula- or gastrula-stage embryo and the process repeated 60-100 times with descendant embryos. In domestic animals the largest number of offspring from one embryo has been 8 calves. Embryos as late as the 64-cell stage in cattle and 120-cell blastocyst in sheep have been used successfully as donors of blastomeres. Recloning has also been done in cattle. Nuclear transfer potentially provides a mechanism for multiplication and production testing of clonal lines, a method for rapid genetic improvement and rapid propagation of a selected genotype. Unfortunately the present efficiencies of subsequent embryo development, pregnancy and embryo survival are less than normal. This paper reviews variables contributing to reduced efficiency and research to improve nuclear transfer.